Positive to positive reproduction process and copy sheet for use therein



March 14, 1967 J. J. A. ROBILLARD 3,309,198

-POSITIVE TO POSITIVE REPRODUCTION PROCESS AND COPY SHEET FOR USETHEREIN Filed Sept. :50, 1965 2 Sheets-Sheet 1 FIG I f r I FIG. 2

INVENTOR. JEAN J. A. ROBILLARD BY w ATTORNEYS March 14, 1967 r J J A.ROBILLARD 3,309,198

POSITIVE TO POSI'iIVE REPRODUCTION PROCESS AND COPY SHEET FOR USETHEREIN Filed Sept. 30, 1965 2 Sheets-Sheet 2 FIG. 6-

INVENTOR JEAN J.A. ROBILLARD ATTORNEYS United States Patent Ofiice3,309,198 Patented Mar. 14, 1967 3,309,198 POSITIVE TO POSITIVEREPRODUCTION PROCESS AND COPY SHEET FOR USE THEREIN Jean J. A.Robillard, Waltham, Mass. (B.P. 137, Paris XIII, France) Filed Sept. 30,1965, Ser. No. 491,786 17 Claims. (Cl. 96-1) This application is acontinuation-in-part of copend.- ing application Ser. No. 205,600, filedJune 27, 1962, and copending application Ser. No. 182,864, filed Mar.27,1962, now abandoned, and refiled as application Ser. No. 567,684 onJuly 25, 1966.

The present invention relates to a positive to positive reproductionprocess involving the anodic oxidation of a metallic conductive basecontrolled by a photoconductive layer and the catalytic reduction of asemiconductor material.

In my copending US. patent application Ser. No. 205,600, filed June 27,1962, there is disclosed a reproduction process in which theintroduction of a small amount of catalyst into a semiconductorstructure under the application of an electrical field provides astable,

high-contrast record of an original image projected ontothe structure.There is disclosed, as an embodiment of the invention set forth therein,an electrophotographic sandwich structure consisting of, in the ordernamed, an electrode which may be formed of conductive glass, aphotoconductive layer which may be formed of cadmium selenide, zincoxide, or antimony trisulfide, a catalyst layer which acts as a sourceof catalytic ions and may comprise, for example, a thin layer of silveror a silver compound, a recording layer comprising a semiconductormaterial and a second electrode which need not be transparent and maycomprise aluminum foil as an example. The electrodes forming the top andbottom parts of the sandwich structure are connected to an electricalsource.

The sandwich structure of my copending application operates in thefollowing manner: When an image is projected onto the glass electrode,the transparent electrode transmits the radiation so that it falls onthe photoconductive layer rendering those portions on which radiationfalls relatively more conductive than the dark portions of thephotoconductive layer. Thus, different conductivity paths will beproduced in the photoconductive layer according to the distribution ofthe image brightness.

The current intensity will vary according to the conductivity of thephotoconductive layer and hence according to the local brightness of theimage. Catalytic ions from the catalytic layer will .be transported intothe semiconductor layer in numbers corresponding to the current flux ineach area of the surface of the semiconductor layer (the polarity ofelectrodes is determined to transport the ion in the direction from thecatalytic layer to the semiconductor layer). The transport of catalystinto the semiconductor layer triggers a change in state of oxidation ofthe semiconductor, usually an oxide, and there by provides a change incolor. While the described structure of my copending application iscapable of producing good quality images, it is, however, not optimallysuited for photocopy reproduction because of the negative imagegenerally obtained from a positive original.

According to the present invention, a dry process, with quite good speed(sensitivity) is provided in which a substantially permanent positiveimage is formed by having an image projected onto a photoconductivelayer which is then placed in contact with a copy sheet coated with amixture of a semiconductor material and a dry electrolyte and applyingan electric field between the conductive base of the photoconductivelayer and the conductive base of the copy sheet and subsequently passingthe copy sheet under a roller or other electrode coated with a catalystmaterial and generating an electric field through the copy sheet toobtain 21 (preferably positive) reproduction of the original.

. It is, accordingly, an object of this invention to provide apositive-to-positive electro-catalytic reproduction system.

It is another object of the present invention to provide a processwhereby an oxide film is obtained on the conductive surface of a copysheet upon the application of an electric field between the conductivebase. of a photoconductive layer placed in contact with the copy sheetand the conductive base of the copy sheet.

It is also an object of the present invention to provide a photographicprocess wherein catalytic ions are electrically injected in minutequantities into an electrosensitive coating on a copy sheet, theinjection of such ions being controlled by an oxide image formed inselected areas of the copy sheet. 1

Still another object of the present, invention is to provide a copysheet comprising an electrosensitive material and a dry electrolyte,which upon contact with a photoconductive layer exposed to an image,will produce under the actuation of an electric field an oxide image inselected areas of the conductive base of the copy sheet.

Other objects and advantages of the present invention will be apparentfrom the consideration of the following description in conjunction withthe appended drawings, in which:

FIGURE 1 is a diagrammatic illustration of a photoconductive recordingmedium according to the present invention;

FIGURE 2 is a diagrammatic illustration of a photoconductive recordingmedium of this invention exposed to the image of the original to bereproduced;

FIGURE 3 is a diagrammatic illustration of an electrosensitive recordingmedium according to the present invention;

FIGURE 4 is a diagrammatic illustration showing the contacting of thephotoconductive recording medium exposed to the image of the originalwith the electrosensitive recording medium under the application of anelectric field;

FIGURE 5 is a diagrammatic illustration of the passing of theelectrosensitive recording medium between two conductive rollers, theupper roller being coated with a catalyst material, to obtain a positivereproduction of the original according to the present'invention; and

FIGURE 6 is a schematic diagram of a photocopy apparatus according tothe present invention.

Referring now to FIGURE 1, photoconductive recording medium 1 is showncomprising a photoconductive layer 2 deposited on a suitable conductivecarrier 3. The conductive carrier may be aluminum foil or the like.However, other metal foil or conductive material may be used. Metalconductors, such as aluminum, nickel, copper, etc. may also be depositedas a conductive material on a non-conductive plastic or the like to formthe carrier. The purpose of the conductive backing is to facilitateexposure of the photoconductive recording medium 1 to an electric field.

The photoconductive layer 2 may be formed of well known photoconductors,including cadmium sulfide, cadmium telluride, zinc oxide, antimonysulfide, selenium, germanium, lead sulfide, zinc sulfide, etc. All ofthese materials show a change in conductivity or resistivity uponexposure to radiation.

The selected photoconductive material is coated on the conductivecarrier as a dispersion in a binder. The binder holds the particles tothe conductive backing and is believed to aid in insulating theparticles from each other.

Suitable binders include Pliolite (resinous copolymers of butadiene andstyrene), Zytel (resin composed of alcohol-soluble polyamides), andVersalon (polyarnide resin).

The photoconductive layer may be prepared by grinding in a ball mill amixture of the photoconductive material with a binder and a solvent. Themixture is ground until a smooth dispersion is obtained. After thecoating has been prepared, it is coated on foil or other conductivepaper. Any suitable coating process may be utilized, for example, aknife coating machine may be used to coat a layer of the photoconductivematerial onto the carrier. The thickness of the coating might beoptimized in a thickness range varying from 5 microns to as much as 2mils. Generally a thin coating is desirable. The solvent is removed byevaporation and the coating is allowed to dry.

Preferred solvents for preparing the photoconductive coating aretoluene, methyl ethyl ketone and alkanols such as absolute ethanol.However, other conventional solvents of the class of aromatic adaliphatic ketones and alcohols may be employed within the scope of thisinvention.

A specific illustrative but nonlimiting example of a photoconductivecoating will now be given. A mixture of 100 grams of zinc oxide, 8 cc.of Pliolite S-7 (30% solution), 1000 cc. toluene is ball milled fortwenty-four hours. After ball milling, a 10 cc. of 0.1% solution of RoseBengal (light sensitive dye) is added to the mixture. A smoothdispersion is obtained which is spread over the conductive carrier andthe solvent removed by evaporation at room temperature.

The photoconductive recording medium is employed according to thepresent invention by exposure to the image of the original to bereproduced. This will result in a latent image being formed in terms oflocal variations of conductivity in the photoconductive layer asillustrated by FIGURE 2.

After the photoconductive layer 2 has been exposed to the image of theoriginal, it is placed in contact, under the application of an electricfield, with an electrosensitive recording medium which will he describedhereinafter.

Referring to FIGURE 3, the electrosensitive recording medium 4 comprisesan electrosensitive layer 5 containing a mixture of sensitive pigments,usually a semiconductor oxide, and a dry electrolyte deposited on aconductive carrier 6. The conductive carrier may comprise aluminum foilor may be a conductive layer on a reinforcing backing such as plastic orpaper.

The semiconductor material in the electrosensitive layer 5 comprises asemiconductor oxide which upon a change in the state of oxidationproduces a change in color.

A list of different semiconductor oxides that are particularly suitablefor the semiconductor material is given below, together with theirreduced states and the change in color which accompanies the change inthe oxidation State: I

TABLE I Materials In order to bring out the semiconductive properties ofthe semiconductor material it is necessary to dope this material. Thedoping material and procedure will vary somewhat with the oxideconsidered as will be understood by those skilled in the art. The dopingmaterial and procedure is determined from the semiconductors energy bandstructure and other solid state properties.

By way of example the preparation of a doped titanium dioxide will bedescribed in some detail. In this procedure commercially pure titaniumdioxide may be used. This material is readily available and is utilizedin the manu facture of paint, for example. Titanium dioxide of thisgrade will normally be found to have many crystal defects which isdesirable for use in formulating a semiconductor oxide according to thepresent invention. If desired, an X-ray analysis of the titanium dioxideto be used may be made to assure that it has the desirable crystaldefects.

The titanium dioxide is sensitized by doping to introduce impurities inminute quantities. A sodium hydroxide solution may be utilized fordoping the titanium dioxide thus introducing sodium ions as impuritiesin the titanium dioxide crystals.

The following procedure may be followed to prepare the doped oxide.Commercially pure titanium dioxide powder is ball milled forapproximately 24 hours to a grain size of about 1 micron.

The finely divided titanium dioxide powder is then agitated for severalhours in a solution of sodium hydroxide (approximately.l0% This solutionmay be obtained by adding the appropriate amount of sodium hydroxide tothe water in which the finely divided titanium dioxide was milled. Theagitation in the sodium hydroxide solution results in the doping of thetitanium dioxide which may thereafter be dried in a vacuum oven.

Another example of a doped semiconductor oxide material according to thepresent invention is based upon the use of stannic oxide (SnO Thepreparation of a doped stannic oxide is substantially similar to thatpreviously described for titanium dioxide. The doping solution may be 5%sodium acetate solution. 1

As a further example of a doped semiconductor oxide, CeO is doped by aprocess substantially as previously described for titanium dioxide bymixing the material with 10% solution of sodium hydroxide and agitatingthis solution for a period of six hours.

The following procedure may be followed to prepare the electrosensitivecoating comprising the semiconductor oxide and dry electrolyte. Thedoped semiconductor oxide is combined with water and ball milled for 24hours. A homogeneous liquid is obtained. To this liquid there is addedthe following: a minor amount of a humc'ctant, a solution of an ionicsalt which provides conductivity in the coating and a compound whichhelps to form the oxide film on the conductive surface of theelectrosensitive layer. This mixture, which is very viscous, is thenadded slowly with agitation to an organic binder in a suitable solvent.There is obtained a dispersion which is coated on a suitable conductivecarrier. The thickness of the coating is not critical and may vary from5 microns to as much as 2 mils in some instances. Generally a thincoating is desired.

The humectants useful in the electrosensitive composition includeglycerol, sorbitol and other glycols. The ionic salt is preferablyammonium nitrate. Compounds suitable for use in the electrosensitivecomposition to bring about the formation of the oxide film in selectedareas include phosphoric acid, chromic acid, ammonium tartrate, ammoniumcitrate, boric acid, oxalic acid, ammonium phosphate, ammoniumpen-taborate and tartaric acid.

The binders useful in the electrosensitive coating include Zytel (resincomposed of alcohol-soluble polyamides), Pliolite (resinous copolymersof butadiene and styrene) and Versalon (polyamide resin).

Preferred solvents for preparing the electrosensitive coating aretoluene, methyl ethyl ketone, and alkanols such as absolute ethanol.However, other conventional solvents may be employed.

A specific illustration example of an electrosensitive composition willnow be given. A mixture of 75 grams of doped tin oxide and 100 grams ofwater are ball milled for twenty-four hours. A homogeneous liquid isobtained. To cc. of this liquid there is added 1 cc. of phosphoric acid,2 cc. of glycerol, and 5 cc. of ammonium nitrate solution). This mixtureis added slowly with agitation to 20 cc. Zytel (15% solution) in 20 cc.ethyl alcohol. There is obtained a dispersion of low viscosity which isspread on a conductive carrier in a uniform layer at a thickness of 1mil.

The foregoing example of the preparation of an electrosensitive coatingused for the electrosensitive recording medium is given by way ofexample only and is subject to considerable modification in accordancewith the knowledge of the art.

Referring to FIGURE 4, the photoconductive recording medium 1, afterexposure to the image of the original, is placed in contact with theelectrosensitive recording medium 4 which is a copy sheet. An electricfield is applied by connecting conductive base 3 and conductive base 6by leads 7 and 8 respectively to an electrical source (electricalpotential difference between about 10 volts and 100 volts), the copysheet being positive with respect to the photoconductive recordingmedium 1. A current will flow between the photocon'ductive layer 2 andthe electrosensitive layer 5 at a rate proportional to the change inconductivity on the photoconduc-tive layer according to the previouslight exposure of the photoconductive recording medium, and a thin oxidefilm 9 will develop on the surface of the conductive layer of the copysheet at the same rate. The oxide film is formed according to the wellknown process of anodic oxidation. The anodizing current is modulated bythe photoconductive layer 1.

Referring now to FIGURE 5, after the oxide film 9 has formed on the copysheet, the sheet is fed between two rollers 10 and 11, the one on thesurface of the sheet being coated with a catalyst and maintained at apositive electrical potential, with respect to the lower roller(potential difference between about 10 volts and 100 volts). The currentthat will flow upon the application of the electric field will bemodulated according to the distribution of the oxide film on theconductive carrier of the copy sheet, that is, the oxide film will offerhigh resistance to the passage of the current and a relatively'smallcurrent will flow in these areas, while higher current density willexist in areas free of the oxide film.

As a result of this current flow, a variable amount of catalyst will betransferred from the catalyst coated roller 10 into the electrosensitivelayer 5, providing darkening proportional to the density of the current.As shown in FIGURE 5, the characteristic of the positive image 12 formedon the copy sheet will be such that to the areas covered by the oxidefilm 9 on the surface of the conductive carrier 6 will correspond whiteareas 120 on the surface of the copy sheet and to the areas free of theoxide film 9 will correspond the dark areas. In this manner a positiveimage of the original is obtained as shown at 12.

The darkening of the copy sheet in those areas free from the oxide filmis due to the change in oxidation state of the oxide in theelectrosensitive coating resulting from the introduction of a smallamount of active catalyst. It is necessary that the catalyst atoms ormolecules be electrically charged in order that they may be transportedinto selected areas of the copy sheet with an electric current. In thepresent invention the catalyst atoms or molecules are ionized.

The catalyst layer deposited on roller 10 may comprise, for example, athin layer of silver or a silver compound. The thin layer can beobtained by vacuum deposition according to the techniques well known inthe art. Methods for so depositing a thinlayer on a. substrate aredisclosed in the text Vacuum Deposition of Thin Films by Holland(published in 1956 in London by Chapman & Hall, ltd.).

The catalyst layer has a very high chemical activity for thicknesseslower than 0.1 micron and as a consequence, they are always oxidized.Furthermore, the bond between the oxygen atoms and the metal atoms ismuch weaker than in a bulk oxide or thick layer of oxide. This is due tothe fact that the resultant of the forces exerted on a particularmolecule of oxide in a direction perpendicular to the film does notbalance the one exerted in a direction parallel to the film. As aresult, if an electric field is applied perpendicular to the film, theoxide molecule will easily dissociate and provide a metal ion which istransportable by the electric current (Langevin theory).

While silver is an appropriate catalyst for use in connection withsemiconductor oxides such as cerium oxide.

titanium dioxide and stannic oxide, other catalysts may be preferredwith other oxides. For example, tantalum oxide may be used with a copperor manganese catalyst, lead oxide may be used with a nickel catalyst. Inaddition, a thallium catalyst may be used with titatnium dioxide, ceriumoxide or germanium dioxide as an alternative to the silver catalyst.

The mechanism of the change in oxidation state due to the introductionof a small amount of catalyst in the semiconductor oxide material in theelectrosensitive layer involves a consideration of the two categories;

In the first'category only the formation of activation centersproportional to the number of catalyst atoms introduced into the systemis involved. These activation centers are invisible and form a latentimage similar in these respects to the one obtained with silver halidephotography. This latent image can be developed by further exposure to aradiation of short wave length such as ultraviolet light.

In the second and preferred category, the change in oxidation state,accompanied by the change in color, takes place spontaneously upon theintroduction of the catalyst under influence of an electric field. Thisinvolves a chain reaction in which the number of oxide moleculesaffected is far greater than the number of atoms of catalyst introducedinto the system.

It will be apparent that the second category process, which does notinvolve further exposure to radiation to develop the latent image, issomewhat preferable. However, the requirement for exposure to furtherradiation is usually met simply by exposure to daylight, fluorescentlight, or other low level illumination with some ultraviolet component.

To present a better understanding of the catalytic reduction involved inthe positive image forming process, the mechanism of the action will beexplained with reference to a specific example, cerium oxide.

The catalytic reduction of the oxide in the semiconductor layer involvedin the positive image forming process is based on the control of thenumber of ions of deviating valency in an ionic crystal by incorporationof impurity ions of a certain type into the lattice.

In the original cerium oxide a small amount of CeO is always presentalong with the predominant CeO This small amount of CeO plays animportant role in the initiation of further reduction. The introductionof catalyst ions such as silver ions into the existing CeO lattice willcontrol the number of Ce ions. The smaller charge of the Ag+ ionsbalances the excess charge of the Ce ions without the simultaneousintroduction of vacancies in the cation lattice.

The defect center may be described as an impurity ca ion of lowerrelative charge on a cation site plus a positive hole bound on aneighboring host cation. In contrast to stoichiometric CeO the silvercontaining material shows a considerable increase in electricalconductivity of the p-type. The silver catalyzed cerium oxide has muchthe same properties as CeO prepared under slightly reducing conditions,but in the latter material, there is no way of estimating or controllingthe concentration of defects. The incorporation of a cation site of animpurity cation of high r charge than the host cation can stabilize alower valence state of the host cation.

In the first category of oxides previously discussed (those requiringdevelopment by exposure to further radiation such as ultraviolet light)the existence of electrical defects in the crystal lattice will providetrapping levels in the energy diagram at different depths under theconduction band. The position of these levels in the energy diagram issuch that the absorption of a quantum of light in the ultraviolet bandwill create free electrons which will be trapped at the existingtrapping centers. As a result, the trapping center will provide a localconcentration of negative charges. In addition to this electrontrapping, an ionic process takes place in the CeO emulsion, where CeOions are displaced from the lattice and are moved foward to the negativetrapping centers where they discharge themselves.

The motion of the ions is of 'two kinds: direct motion from oneinterstitial position in the lattice to another, and the filling up ofthe vacated sites by several ions in the lattice producing new vacantsites so that positive holes migrate. As the development proceeds, moreelectrons are provided to the trapping levels by the absorbed UV lightand the reduced state of oxide is built up accordingly. The oxygen atomsreleased are diffused to the surface or remain interstitially.

In the second category of materials, the development of the image takesplace immediately upon the introduction of the catalyst into the oxidelayer without absorption of UV light for the development. The mechanismis as follows: The trapping of an electron by the trapping centers asmentioned previously, tends to hasten the diffusion of ions away fromtheir original sites and leave an equivalent number of cations adjacentto the reduced ion. These cations with anion vacancies are added to thereduced ion neutralizing the trapped electron, and so on. Thisrepresents the autocatalytic phase of the reaction which is responsiblefor amplification and self-development of the image.

A study of the reduction mechanism of different oxides, together withtheir band structure and semiconductive properties will determinewhether an autocatalytic image forming reaction is possible. When thispossibility exists, emulsions can be made which will not require UVexposure for development.

Examples of oxides which do not require any ultraviolet development areSnO Ta O and TeO The photoconductive recording medium of FIGURE 1 andthe electrosensitive recording medium of FIGURE 3 are particularlyuseful in photocopy applications and may be utilized in an apparatussuch as that illustrated in FIG- URE 6, for example.

Photocopy apparatus 13 comprises a lens 14 for focusing an image of adocument 15 or other subjects to be reproduced. The image of a documentis focused by lens 14 under illumination from light sources 16 and 17onto a photoconductiv belt 18 which is formed of a sheet as described inFIGURE 1. The photoconductive belt 18 is driven by rollers 19 and 20. Acopy sheet 22, which is a sheet as described in FIGURE 3, is contactedwith the photoconductive belt 18 under the pressure of rollers 19 and21. These rollers 19 and 21 are electrically connected by leads 23 and24 to a source of electrical potential 25 providing the necessaryelectrical field for the formation of an oxide film in selected areas onthe surface of the conductive base of copy sheet 22.

After passing between rollers 19 and 21, copy sheet 22 is passed betweenrollers 26 and 27 which are electrically connected by leads 28 and 29 toa source of electrical potential 30 providing the necessary current forthe formation of the positive image 31 of the original on copy sheet 22.The conductive roller 27 is coated with a solid catalyst which migratesinto the copy sheet 22 upon the application of an electric field.

The positive reproduction 31 of the original 15 is trans- 8 ferredoutside the machine by belt 32 driven by pulleys 33 and 34.

The latent photoconductive image provided on belt 18 after exposure tothe image can be erased 'by exposure to infrared lamp 35 and the belt isthen ready for a new exposure. The absorption of the infrared light bythe belt 18 provides enough energy to release the electrons from thetrapping centers through vibration of the lattice. The absorption oflight by the belt 18 upon exposure to the image promotes the trapping ofelectrons and the infrared light releases these electrons.

The operation of the photocopy apparatus of FIGURE 6 is as follows. Animage is projected by lens 14 onto a photoconductive belt 18 for theexposure thereof. After the formation of a latent image in terms oflocal variations of conductivity in the belt 18, the belt is contactedwith copy sheet 22 under the pressure of rollers 19 and 21 and anelectric field is applied between the copy sheet 22 and the belt 18, thecopy sheet being positive with respect to the belt. The flow of acurrent between belt 18 and copy sheet 22 will result in the formationof an oxide film in selected areas on the surface of the conductive baseof the copy sheet. The copy sheet is then fed between conductive rollers26 and 27, roller 27 being coated with a solid catalyst which willmigrate into selected areas of the copy sheet upon the application of anelectric field between rollers 26 and 27, thereby providing a positiveimage of the original on the copy sheet. The image formed on belt 18 iserased by exposure to infrared lamp 35.

The theoretical explanations of the processes involved in the presentinvention have been explained as they are presently understood and arebelieved correct. However the advantages, operability or scope of theinvention is not predicated on the accuracy of such explanations butupon the actual operation of the disclosed devices, apparatus andprocesses.

Other variations and embodiments will be apparent to those of skill inthe art and it is accordingly desired that the scope of the inventionnot be limited to those embodiments particularly illustrated orsuggested, but that the scope of the invention be defined by referenceto the appended claims.

What is claimed is:

1. A process of producing a substantially permanent visible photographicrecord characteristic of a radiation image which comprises (a) exposingto said image the surface of a photoconductive recording mediumsupported on an electrically conductive backing, said photoconductiverecording medium comprising a layer of photoconductive material whichexhibits a change in conductivity in those areas of said photoconductiverecording medium exposed to said radiation image and forming as a resultof exposure of said photoconductive recording medium to said image alatent image in terms of local variations of conductivity in saidphotoconductive recording medium, (b) contacting the surface of saidphotoconductive recording medium with the surface of an electrosensitivemedium comprising an electrolyte and an oxidizable metallic electricallyconductive backing, (c) impressing an electric field through saidphotoconductive recording medium and said electrosensitive medium toobtain a thin highly resistive oxide film in selected areas on thesurface of the electrically conductive backing of said electrosensitivemedium, said current flow between said photoconductive recording mediumand said electrosensitive oxidizable medium being substantiallyproportional to the change in conductivity in said photoconductiverecording medium according to the previous exposure of said medium tosaid radiation image, (d) placing a solid metal catalyst source incontact with the surface of a semiconductor material layer having atleast two oxidation states of substantially contrasting color with thelower oxidation state being intensely colored, said semiconductormaterial being substantially in a higher oxidation state and saidsemiconductor material being doped with a donor impurity compatible withthe energy band of said semiconductor material, said semiconductormaterial layer being also in contact with said oxide film on saidelectrically conductive backing and (e) impressing an electric fieldthrough said solid metal catalyst source, said semiconductor materiallayer and said electrically conductive backing on which said oxide filmis formed to inject metal ions of said cat alyst into said semiconductormaterial layer, said current that flows upon the impression of saidelectric field being modulated according to the distribution of saidoxide film on the surface of said electrically conductive backing, saidoxide film offering a higher resistance to the passage of current andsaid metal ions into said semiconductor material layer than those areason the surface of said electrically conductive backing free of saidoxide film, whereby the injection of said metal ions into saidsemiconductor material layer causes said semiconductor material toconvert into a lower oxidation state in selected areas of said layerthereby producing a photographc record of the image.

2. A process according to claim 1 wherein the photoconductive materialis selected from the group consisting of cadmium sulfide, cadmiumtelluride, zinc oxide, antimony sulphide, zinc sulphide, lead sulphide,selenium, and germanium.

3. A process according to claim 1 wherein said dry electrolyte comprisesan ionic salt, a humectant and a com-pound selected from the classconsisting of phosphoric acid, chromic acid, ammonium tartrate, ammoniumcitrate, boric acid, oxalic acid, ammonium phosphate, ammoniumpentaborate and tartaric acid.

4. A process according to claim 1, wherein the ions of solid metalcatalyst are selected from the group consisting of silver, copper,manganese and thallium.

5. A process according to claim 4 wherein the catalyst comprises silver.

6. A process according to claim 1, wherein the semiconductor material isa semiconductor oxide.

7. A process according to claim 6 wherein said semiconductor oxide istitanium dioxide and said solid metal catalyst is silver.

8. A process according to claim 1, which includes the additional step ofexposing said semiconductor material layer and the metal ions therein toa light source rich in ultraviolet.

9. A process for the positive reproduction of a light image on a copysheet which comprises: (a) exposing to said image the surface of aphotoconductive recording medium supported on an electrically conductivebacking, said photoconductive recording medium comprising a layer ofphotoconductive material which exhibits a change in conductivity inthose areas of said photoconductive recording medium exposed to saidlight image and forming as a result of exposure of said photoconductiverecording medium to said image a latent image in terms of localvariations of conductivity in said photoconductive recording medium, (b)contacting the surface of said photoconductive recording medium with thesurface of a copy sheet comprising a semiconductor oxide having at leasttwo oxidation states of substantially contrasting color with the loweroxidation state being intensely colored, an electrolyte and anoxidizable electrically conductive backing, said semiconductor oxidebeing substantially in a higher oxidation state and said semiconductoroxide being doped a donor impurity compatible with the energy band ofsaid semiconductor oxide, (c) impressing an electric field through saidphotoconductive recording medium and said copy sheet to obtain a thinhighly resistive oxide film in selected areas on the surface of theelectrically conductive backing of said copy sheet, said current flowbetween said photoconductive recording medium and said copy sheet beingsubstantially proportional to the change in conductivity in saidphotoconductive recording medium according to the previous exposure ofsaid medium to said light image, ((1) placing a solid metal catalystsource in contact with the surface of said copy sheet, and (e) im- Itpressing an electric field through said catalyst source and said copysheet to inject metal ions of said catalyst into said semiconductoroxide in said copy sheet, said metal ions being selected from the classconsisting of silver, copper, manganese and thallium, said current thatflows upon the impression of said electric field being modulatedaccording to the distribution of said oxide film on the surface of saidelectrically conductive backing, said oxide film offering a higherresistance to the passage of current and said metal ions into saidsemiconductor oxide than those areas on the surface of said electricallyconductive backing free of said oxide film, whereby the injection ofsaid metal ions into said semiconductor oxide causes said semiconductoroxide to convert to a lower oxidation state in selected areas of saidcopy sheet thereby producing a positive image of the original on saidcopy sheet.

10. An electrosensitive coating comprising a semiconductor oxide havingtwo oxidation states of different color, said oxide being substantiallyin the higher state of oxidation and doped with an impurity in aquantity to render the coating sensitive to the introduction of arelatively small number of ions of a solid catalyst upon the applicationof an electric field to change said oxide by catalytic action from thehigher to a lower state of oxidation, and a dry electrolyte mixed withsaid semiconductor oxide and dispersed in an organic binder.

11. A coating according to claim 10 wherein the oxide comprises titaniumdioxide.

12. A coating according to claim 10 wherein the oxide comprises ceriumdioxide.

13. A copy sheet comprising a mixture of a semiconductor oxide and a dryelectrolyte, said oxide having two oxidation states of different color,said oxide being substantially in the higher state of oxidation anddoped with an impurity in a quantity to render the coating sensitive tothe introduction of a relatively small number of ions of a solidcatalyst upon the application of an electric field to change said oxideby catalytic action from the higher state of oxidation to a lower stateof oxidation, said semiconductor oxide and dry electrolyte beingsupported on an electrically conductive backing containing a materialcapable of undergoing anodic oxidation.

14. An electrosensitive recording medium comprising a semiconductoroxide doped with a donor impurity, an electrolyte, an organic binder anda oxidizable electrically conductive backing, said semiconductor oxidehaving at least two oxidation states of substantially contrasting colorwith the lower oxidation state being intensely colored, and said oxidebeing substantially in the higher oxidation state and said electrolytecomprising an ionic salt, a humectant and a compound selected from theclass consisting of phosphoric acid, chromic acid, boric acid, oxalicacid, tartaric acid, ammonium phosphate, ammonium citrate, ammoniumtartrate and ammonium pentaborate.

15. A recording medium according to claim 14 wherein the semiconductoroxide is selected from the group con sisting of TiO smo and CeO 16. Arecording medium according to claim 14 wherein said donor impurity aresodium ions.

17. A copy sheet comprising an electrosensitive layer on a contiguousoxidizable electrically conductive backing, said electrosensitive layercomprising: (1) a semiconductor oxide doped with a donor impurity in aquantity suflicient to render said oxide sensitive to the introductionof a relatively small quantity of metal ions of a solid metal catalystupon the application of an electric field, said oxide having at leasttwo oxidation states of substantially contrasting color with the loweroxidation state being intensely colored and said oxide beingsubstantially in the higher oxidation state, (2) an organic binder, (3)a dry electrolyte comprising an ionic salt, a humectant and a compoundselected from the class consisting of a phosphoric acid, chromic acid,boric acid, oxalic acid, tartaric acid, am-

1 1 1 2 monium phosphate, ammonium citrate, ammonium tar- 3,106,156 10/1963 Reithel 96-1 trate and ammonium pentaborate. 3,142,562 7/ 1964Blake 204-48 3,152,903 10/1964 Shepard et a1 96- 64 References Cited bythe Examiner UNITED STATES PATENTS 3,041,166 6/1962 Bardeen 96-13,082,085 3/1963 Miller et a1 961 5 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

1. A PROCESS OF PRODUCING A SUBSTANTIALLY PERMANENT VISIBLE PHOTOGRAPHICRECORD CHARACTERISTIC OF A RADIATION IMAGE WHICH COMPRISES (A) EXPOSINGTO SAID IMAGE THE SURFACE OF A PHOTOCONDUCTIVE RECORDING MEDIUMSUPPORTED ON AN ELECTRICALLY CONDUCTIVE BACKING, SAID PHOTOCONDUCTIVERECORDING MEDIUM COMPRISING A LAYER OF PHOTOCONDUCTIVE MATERIAL WHICHEXHIBITS A CHANGE IN CONDUCTIVITY IN THOSE AREAS OF SAID PHOTOCONDUCTIVERECORDING A MEDIUM EXPOSED TO SAID RADIATION IMAGE AND FORMING AS ARESULT OF EXPOSURE OF SAID PHOTOCONDUCTIVE RECORDING MEDIUM, TO SAIDIMAGE A LATENT IMAGE IN TERMS OF LOCAL VARIATIONS OF CONDUCTIVITY INSAID PHOTOCONDUCTIVE RECORDING MEDIUM, (B) CONTACTING THE SURFACE OFSAID PHOTOCONDUCTIVE RECORDING MEDIUM WITH THE SURFACE OF ANELECTROSENSITIVE MEDIUM COMPRISING AN ELECTROLYTE AND AN OXIDIZABLEMETALLIC ELECTRICALLY CONDUCTIVE BACKING, (C) IMPRESSING AN ELECTRICFIELD THROUGH SAID PHOTOCONDUCTIVE RECORDING MEDIUM AND SAIDELECTROSENSITIVE MEDIUM TO OBTAIN A THIN HIGHLY RESISTIVE OXIDE FILM INSELECTED AREAS ON THE SURFACE OF THE ELECTRICALLY CONDUCTIVE BACKINGSAID PHOTOCONDUCTIVE MEDIUM, SAID CURRENT FLOW BETWEEN SAIDPHOTOCONDUCTIVE RECORDIG MEDIUM AND SAID ELECTROSENSITIVE OXIDIZABLEMEDIUM BEIN GSUBSTANTILLY PROPORTIONAL TO THE CHANGE IN CONDUCTIVITY INSAID PHOTOCONDUCTIVE RECORDING MEDIUM ACCORDING TO THE PREVIOUS EXPOSUREOF SAID MEDIUM TO SAID RADIATION IMAGE, (D) PLACING A SOLID MTALCATALYST SOURCE IN CONTACT WITH THE SURFACE OF A SEMICONDUCTOR MATERIALLAYER HAVING AT LEAST TWO OXIDATION STATES OF SUBSTANTIALLY CONTRASTINGCOLOR WITH THE LOWER OXIDATION STATE BEING INTENSELY COLORED, SAIDSEMICONDUCTOR MATERIAL BEING SUBSTANTIALLY IN A HIGHER OXIDATION STATEAND SAID SEMICONDUCTOR MATERIAL BEING DOPED WITH A DONOR IMPURITYCOMPATIBLE WITH THE ENERGY BAND OF SAID SEMICONDUCTOR MATERIAL, SAIDSEMICONDUCTOR MATERIAL LAYER BEING ALSO IN CONTACT WITH SAID OXIDE FILMON SAID ELECTRICALLY CONDUCTIVE BACKING AND (E) IMPRESSING AN ELECTRICFIELD THROUGH SAID SOLID METAL CATALYST SOURCE, SAID SEMICONDUCTORMATERIAL LAYER AND SAID ELECTRICALLY CONDUCTIVE MATERIAL SAID OXIDE FILMIS FORMED TO INJECT METAL IONS OF SAID CATALYST INTO SAID SEMICONDUCTORMATERIAL LAYER, SAID CURRENT THAT FLOWS UPON THE IMPRESSION OF SAIDELECTRIC FIELD BEING MODULATED ACCORDING THE THE DISTRIBUTION OF SAIDOXIDE FILM ON THE SURFACE OF SAID ELECTRICALLY CONDUCTIVE BACKING, SAIDOXIDE FILM OFFERING A HIGHER RESISTANCE OT THE PASSAGE OF CURRENT ANDSAID METAL IONS INTO SAID SEMICONDUCTOR MATERIAL LAYER THAN THOSE AREASON THE SURFACE OF SAID ELECTRICALLY CONDUCTIVE BACKING FREE OF SAIDOXIDE FILM WHEREBY THE INJECTION OF SAID METAL IONS INTO SAIDSEMICONDUCTOR MATERIAL LAYR CAUSES SAID SEMICONDUCTOR MATERIAL TOCONVERT INTO A LOWER OXIDATION STATE IN SELECTED AREAS OF SAID LAYERTHEREBY PRODUCING A PHOTOGRAPHIC RECORD OF THE IMAGE.
 10. ANELECTROSENSITIVE COATING COMPRISING A SEMICONDUCTOR OXIDE HAVING TWOOXIDATION STATES OF DIFFERENT COLOR, SAID OXIDE BEING SUBSTANTIALLY INTHE HIGHER STATE OF OXIDATION AND DOPED WITH AN IMPURITY IN A QUANTITYTO RENDER THE COATING SENSITIVE TO THE INTRODUCTION OF A RELATIVELYSMALL NUMBER OF IONS OF A SOLID CATALYST UPON THE APPLICATION OF ANELECTRIC FIELD TO CHANGE SAID OXIDE BY CATALYTIC ACTION FROM THE HIGHERTO A LOWER STATE OF OXIDATION, AND A DRY ELECTROLYTE MIXED WITH SAIDSEMICONDUCTOR OXIDE AND DISPERSED IN AN ORGANIC BINDER.